Disc drives of the type known as "Winchester" disc drives, or hard disc drives, are well known in the industry. Such disc drives magnetically record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless DC spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 10,000 RPM.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically consist of an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by head suspensions or flexures.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator bearing housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies, and supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. On the side of the actuator bearing housing opposite to the coil, the actuator bearing housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted. When controlled DC current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator bearing housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorentz relationship. As the actuator bearing housing rotates, the heads are moved radially across the data tracks along an arcuate path.
Disc drives of the current generation are included in desk-top computer systems for office and home environments, as well as in laptop computers which are used wherever their users happen to take them. Because of this wide range of operating environments, the computer systems, as well as the disc drives incorporated in them, must be capable of reliable operation over a wide range of ambient temperatures.
Furthermore, laptop computers in particular can be expected to be subjected to large amounts of mechanical shock as they are moved about. It is common in the industry, therefore, that disc drives be specified to operate over ambient temperature ranges of from, for instance, -5.degree. C. to 60.degree. C., and further be specified to be capable of withstanding mechanical shocks of 100 G or greater without becoming inoperable.
One of the areas of disc drive design which is of particular concern when considering ambient temperature variations and mechanical shock resistance is the system used to mount the discs to the spindle motor. During manufacture, the discs are mounted to the spindle motor in a temperature- and cleanliness-controlled environment. Once mechanical assembly of the disc drive is completed, special servo-writers are used to prerecord servo information on the discs. This servo information is used during operation of the disc drive to control the positioning of the actuator used to move the read/write heads to the desired data location in a manner well known in the industry. Once the servo information has been recorded on the discs, it is assumed by the servo logic that the servo information, and all data subsequently recorded, are on circular tracks that are concentric with relation to the spin axis of the spindle motor. The discs, therefore, must be mounted to the spindle motor in a manner that provides sufficient clamping force to prevent shifting of the discs relative to the spindle motor due to differential thermal expansion of the discs and motor components over the specified temperature range, or due to mechanical shock applied to the host computer system.
Several systems for clamping of the discs to the spindle motor have been described in U.S. Pat. Nos., including U.S. Pat. No. 5,528,434, issued Jun. 18, 1996, U.S. Pat. No. 5,517,376, issued May 14, 1996, U.S. Pat. No. 5,452,157, issued Sep. 19, 1995, U.S. Pat. No. 5,333,080, issued Jul. 26, 1994, U.S. Pat. No. 5,274,517, issued Dec. 28, 1993 and U.S. Pat. No. 5,295,030, issued Mar. 15, 1994, all assigned to the assignee of the present invention and all incorporated herein by reference. In each of these incorporated disc clamping systems, the spindle motor of the disc drive includes a disc mounting flange extending radially from the lower end of the spindle motor hub. A first disc is placed over the hub during assembly and brought to rest on this disc mounting flange. An arrangement of disc spacers and additional discs are then alternately placed over the spindle motor hub until the intended "disc stack" is formed. Finally, some type of disc clamp is attached to the spindle motor hub which exerts an axial clamping force against the uppermost disc in the disc stack. This axial clamping force is passed through the discs and disc spacers and squeezes the disc stack between the disc clamp and the disc mounting flange on the spindle motor hub.
From the above description, it would appear that the only element that would need to be considered when designing a disc clamping system would be the disc clamp, with any requirement for additional clamping force being met by an increase in the strength of the disc clamp. However, with the industry trend of size reduction in the overall disc drive, the size of various components within the disc drive has also been reduced, including the thickness of the discs. As the discs have grown thinner, the amount of clamping force that can be applied to the discs without causing mechanical distortion of the discs has also fallen. That is, due to inescapable tolerance variation in the flatness of the disc mounting flange on the spindle motor, the discs themselves and the disc spacers between adjacent discs, as well as the yield strength of the disc material, only a finite amount of axial clamping force can be applied to the inner diameters of the discs before the desired flatness of the disc surfaces is lost.
One type of disc clamp which is used extensively in the industry is the so-called "spring-type" clamp. A spring-type clamp is typically formed of sheet material stamp-formed to provide both mounting and force-application features, and commonly consists of three major portions: 1) a central mounting portion; 2) a spring portion extending radially outward from the central mounting portion and; 3) a contact portion adjacent the outer diameter of the spring-type clamp.
The central mounting portion, also sometimes referred to as a web, typically includes one or more screw holes through which machine screws are inserted into corresponding tapped holes in the upper surface of the spindle motor hub. It is also typical for the web to include an arrangement of tooling holes, aligned with corresponding tooling holes in the upper surface of the spindle motor hub, which are engaged by an assembly tool to maintain the relative position of the spindle motor and disc clamp while the screws used to mount the disc clamp are tightened.
The radially extending spring portion is commonly formed at an angle to the plane of the central mounting portion of the disc clamp, and acts, when the web is displaced into contact with the top of the spindle motor hub, similarly to a "belleville" spring to determine the amount of clamping force applied to the top surface of the uppermost disc in the disc stack.
The contact portion of a typical spring-type disc clamp is a circumferentially formed corrugation at the outermost extent of the spring portion. The corrugation is first formed downward, toward the disc surface, and then back upward, thus forming a contact portion which is substantially circular in section at a fixed diameter from the spin axis of the disc stack, and producing a perimeter wall at the outer extreme of the disc clamp.
While spring-type disc clamps have been seen which employ a single, centrally located mounting screw, it is much more common to utilize a plurality of screws evenly spaced about a diameter just inside the outer diameter of the web portion of the disc clamp. The use of multiple mounting screws placed close to the spring portion provides greater overall clamping force than a single central mounting screw, given the same configuration of the remainder of the disc clamp.
One typical drawback to the use of multiple mounting screws for the disc clamp is uneven distribution of the clamping force between locations radially aligned with the mounting screws and those portions of the disc clamp contact surface lying circumferentially between adjacent screw locations. For example, such a disc clamp often produces the greatest contact surface stress on the top disc surface at the same angular locations as the screw positions. This is especially true in disc clamp designs that have a web that is substantially free of secondary holes, openings or cutouts.
In disc clamp designs that do have secondary openings, the openings can direct the load concentrations away from the angular position of the screws to angular locations at the contact surface other than at the angular positions of the screws.
In any case, if the stress at the contact surface is localized, those locations at the contact surface where the load stress is not concentrated can experience a significantly lower contact force, in some clamp designs even approaching zero force.
The end result is that the circumferentially varying contact surface stress introduces an associated circumferential waviness into the uppermost disc in the disc stack that is most severe near the inner diameter clamping area, but which may possibly extend over the entire disc surface. Such waviness in the disc surface can have a major impact on the flying characteristics of the heads, particularly if the wave length of the disc distortion is of the same scale as the length of the slider used to support the read/write transducer.
Previously incorporated U.S. Pat. No. 5,333,080, U.S. Pat. No. 5,528,434 and U.S. Pat. No. 5,517,376 are each specifically directed to improving the distribution of clamping force provided by spring-type disc clamps using multiple mounting screws, and the resultant reduction in mechanical distortion of the discs, particularly the top disc, in the disc stack. U.S. Pat. No. 5,333,080 also requires, however, the inclusion of a shim between the disc clamp and the spindle motor hub, which increases part count in the disc drive, adds to assembly complexity, and adds an otherwise non-functional element to the overall vertical height of the disc stack.
Similarly, U.S. Pat. No. 5,517,376 requires the inclusion of specially formed components and additional assembly adjustments in order to achieve even distribution of disc clamping forces.
U.S. Pat. No. 5,528,434 achieves even distribution of disc clamping force by including an additional circumferential bend in the disc clamp between the central mounting portion and the circular contact surface at the outer extent of the spring member. While each of these prior art disc clamping systems achieves improvement in the distribution of disc clamping forces, they each require either the inclusion of additional components--and potentially increases in the height requirement of the clamping system--or an increase in the complexity of the forming of the disc clamp after the unformed disc clamp is excised from flat spring stock.
A need clearly exists, therefore, for a disc clamping system which provides an even distribution of the clamping force applied to the disc stack to prevent mechanical distortion of the discs.